This invention relates to a gas laser tube, and more particularly to a structure for holding and fixing a cathode thereof.
A gas laser tube such as helium neon gas laser tube utilizes an aluminum cold cathode. This is usually of cylindrical shape and longitudinally overlaps within a glass envelope a capillary discharge tube. In U.S. Pat. No. 4,311,969 , a cylindrically shaped aluminum cathode has an extending cylindrical snout receivable within a central opening of a nickel-iron alloy end plate for an envelope. The end plate is designed with a central axially extending neck section defining the central opening. The axial length of the snout of the cathode is greater than the axial length of the opening in the neck section of the end plate so that the end of the snout can be peened over the neck section to effect both electrical contact by way of the end plate and at the same time be properly mechanically supported.
However, the difference in thermal expansion coefficient between the aluminium and nickel-iron alloy affects the reliability for electrical connection there-between. Temperature changes occur when an optical resonator mirror is sealed in a mirror holder with low-melting glass, or when the whole laser tube is baked in an oven in order to exhaust gas from the envelope.
Aluminum has a thermal expansion coefficient of more than three times higher than that of the nickel-iron alloy. For example, the thermal expansion coefficient of a so-called 426 alloy is 90.times.10.sup.-7 /C..degree. at 30.degree..about.380.degree. C., while the thermal expansion coefficient of Al is 29.times.10.sup.-6 /C..degree. at 0.degree..about.600.degree. C. When heated to a high temperature an, aluminum cathode therefore tends to expand outward, but because the end plate does not expand as much as the cathode, an internal force is exerted on the contact point. As aluminum is much softer than the 426 alloy, the point on which the force is exerted is apparently deformed in the inward direction compared to other locations where no force is exerted. When the temperature falls from this state to the normal operating temperature (several tens of degrees centigrade), the cathode is compressed remarkably toward the inside, and the alignment between the cathode and the internal surface of the end plate is degraded.
As mentioned above, the cathode fixing structure is unsatisfactory in that thermal hysteresis weakens the alignment and electric contact of the cathode, and if the tube is subjected to vibrations or shocks, electric discharge is instantaneously suspended.